The present invention relates to a probe system that includes a distributed resistor to compensate for the inherent transmission loss of the probe system when connecting a circuit board component to a test instrument. A key feature of the invention is the distributed resistor that decreases attenuation substantially in proportion with the square root of increasing frequency over a range of frequencies.
Electrical test probe systems typically interface test instruments to a piece of equipment-under-test (hereinafter referred to as a “EUT”), such as circuit boards, to measure the electrical signal on one or more components on the EUT. Circuit boards components (hereinafter referred generally as “components”) typically include one or more integrated circuits (hereinafter “IC devices”), each IC device having a plurality of leads, legs, pins, or combinations of leads, legs, and pins (hereinafter referred to as “leads”). Other circuit board components may include resistors, capacitors, and through-hole pins, for example. Typical test instruments include, for example, oscilloscopes, spectrum analyzers, or other measuring, monitoring, diagnostic, or signal-processing devices (hereinafter “test instruments”).
Because probe systems establish electrical connections between test instruments and EUTs for the purpose of observing component activity without influence, the ideal probe system would be easy to connect to the EUT, not load the signal source, and transfer the signal accurately. In other words, the ideal probe system would have infinite input resistance, zero input capacitance, and have a flat transfer response over the entire frequency range. In implementation, however, any probe system will have some loading because the test instrument needs to draw some current; the probe system transfer response will have some variation over the frequency ranges because of stray capacitances, inductances, and losses in the probe system.
The physical geometry and configuration of the probe system affects loading and distortion. A probe system, at a minimum, consists of at least one probe tip (e.g. a single probe tip for a single ended probe and two probe tips for a differential probe), a cable, and a connector. Additional probe system components might include a probing head and/or an amplifier. Cable generally includes a metal conductor that has an associated, non-zero, resistance; this results in dissipated signal power, termed “loss.” This loss increases substantially in proportion with the cable length. Additionally, cables generally may include several individual strands, or filaments, braided together with each filament surrounded by a dielectric that also leaks (e.g. has a loss).
In a low-impedance probe system, the cable and other transmission lines in the probe system have a loss that is substantially proportional to the square root of the frequency. The probe tip has an inductance that is primarily a function of the probe tip and/or reference connection length and capacitances that are generally parasitic. These stray inductances and capacitances will create a peak in the frequency response, followed by a drop, as the frequency gets higher. The inductance and capacitance will limit the ability of the probe tip to accurately transfer the signal being measured, and limit the maximum frequency of operation. The probe tip is generally constructed to minimize the inductances and capacitances in order to maximize the frequency range.
Cable loss, therefore, can be quantified as substantially equal to:
L=k•√f; where Loss (L) is measured in decibels (dB) and k is a constant determined by the measured loss at a specified frequency (f).
The loss of a low-impedance (Z0) probe at very low frequencies may be expressed as:L=Z0•log {(Rt+50)/50}
in a 50-ohm system where Rt is the tip resistor.
To compensate for the inherent cable loss in the probe system, attempts have been made to “tune” the probe for a given frequency range. The peaking in the probe tip can act to partially compensate for the cable loss. Within the target frequency range, the ideal probe will exhibit an improved frequency response to the intercepted signal.
One significant problem with known probe systems is an inherent mismatch of the peaking frequency response of the probe tip and the loss associated with transmission loss in the cable connecting the probe tip to the test instrument. Any probe system will have a natural peaking. Peaking refers to the condition where an input inductance resonates with the input capacitance. Attempts to adapt the physical geometry and configuration of other probe components to compensate for cable loss are inadequate. In addition, although probe tip geometry can be changed try to get the “peaking” to match the cable loss, modifying the probe tip and tip length to tune peaking to cable loss is imperfect because the frequency-related variation in the probe tip does not directly correlate with frequency-related loss in the cable.